1. Field of the Invention
This invention relates to zener diodes and, more specifically, to zener diodes having a relatively high reverse breakdown voltage.
2. Brief Description of the Prior Art
Zener diodes are a well known electronic components which have the property of acting as normal diodes in a circuit with conduction taking place in only one direction. However, if the voltage across the zener diode in the reverse direction reaches some known value, the diode will break down and conduct in the reverse direction. In most power integrated circuit designs, zener diodes are utilized for snubbing stacks and for protection of voltages across terminals. At present, low voltage NPN zener diode structures, generally with a breakdown voltage in the range of about 6 volts, are used to provide the above-described protection. The zener diode junction is formed by the base-to-emitter breakdown of an NPN device.
A disadvantage of the above-described structure is the low zener diode breakdown voltage. In many instances, a zener diode with a larger breakdown voltage, if available, would result in the need to implement fewer components. For example, gate-to-source voltage V.sub.gs protection zener diodes require that such protection be close to, but not exceed 20 volts. With 6.5 volt zener diodes, there would be a requirement for two such zener diodes and several base-to-emitter voltage V.sub.be to achieve the breakdown voltage. Another example is a low-side drain-to-gate snub stack where large voltages are required. There, the voltage involved is, for example, 60 volts, thereby requiring a large number of zener diodes for protection. This translates into the requirement that a large amount of die area must be used for the zener diodes alone, which is not economical. It is therefore apparent that a zener diode with a higher breakdown characteristic which also requires no additional die area or, at most, minimally more die area, would be highly desirable.
Another disadvantage with the NPN zener diode is that it requires additional process extensions in the fabrication process. Any process which would reduce the number of processing steps generally provides an economic advantage and this is always sought.
A further disadvantage is the problem of premature breakdown, also known as "punch through", of the NPN zener diode base region to the substrate. This occurs when the base region is brought up to a high enough potential to cause undesirable breakdown between the zener diode and the substrate.